1. Field of the Invention
This invention relates to a magnetic bias apparatus for a magnetooptical disk drive apparatus in which information is recorded on and erased from a recording medium using the magnetooptical effect.
2. Description of the Prior Art
The technique of magnetooptically recording information on a recording medium such as a disk has drawn great attention. A magnetooptical recording apparatus using this technique is provided with a magnetic bias apparatus which is disposed in a position opposite to an optical head.
FIG. 11 diagrammatically recording apparatus. The prior art magnetooptical recording apparatus. The apparatus of FIG. 11 comprises a motor 1a for rotating a magnetooptical disk 1, an optical head 2, and a magnetic bias apparatus A which faces the optical head 2 through the disk 1. The optical head 2 includes an object lens 2a, and a semiconductor laser device (not shown). The magnetic bias apparatus A has a permanent magnet 3, and a coil 4 which is arranged above the magnet 3 (i.e., above the N-pole of the magnet). The magnet 3 is rotatable about an axis 3a. When data erasing is performed. The S-pole of the magnet 3 is directed as shown in FIG. 11 to face the disk 1 so as to produce a magnetic field of a predetermined direction, and a light beam emitted from the optical head 2 is converged onto the disk 1, thereby erasing data stored on the disk 1. When data is to be recorded on the disk 1, the coil 4 is driven to rotate the magnet 3 about the axis 3a by a half turn, so that the N-pole faces the disk 1 and a magnetic field of a direction opposite to the predetermined direction is applied to the disk 1. Then, a light beam modulated in accordance with the data to be recorded is converged onto the disk 1 to record the data on the disk. Alternatively, the relationships of the directions of the magnetic fields may be inverted, in other words, the magnet 3 may be positioned so that when data is to be erased the N-pole faces the disk 1 and when data is to be recorded the S-pole faces the disk 1.
It is desirable that a magnetooptical recording apparatus should be as small and as thin as possible. For this reason, it is necessary for a magnetic bias apparatus also to be made as thin as possible. In an apparatus having the structure shown in FIG. 11, the coil 4 for reversing the direction of the magnetic field is positioned above the permanent magnet 3, and therefore the magnetic bias apparatus cannot be thinly constructed.
FIG. 12 illustrates another magnetic bias apparatus. In the apparatus shown in FIG. 12, the coil 4 and the permanent magnet 3 are juxtaposed in parallel so that the overall height of the apparatus can be decreased. Although the apparatus shown in FIG. 12 can solve the above-mentioned problem, it has another problem in that the coil 4 generates the magnetic flux as shown in FIG. 12, and therefore the rotating force f which is caused by the magnetic field of the coil 4 and acts on the magnet 3 is extremely small, resulting in that a significant amount of time is required for rotating the magnet 3 or the response characteristic of the magnetic bias apparatus is poor.